Measurement of soluble protein degradation in the rumen

Twenty-five feeds were screened for buffer soluble N (BSN) and its constituent protein N, peptide N, amino acid N and ammonia N. In vitro degradation rates of buffer soluble protein were determined in 11 of these feeds. To estimate rumen in vivo degradation rates, buffer soluble protein from peas or cold pressed rapeseed cake was also given as a pulse dose, together with a liquid marker (CoEDTA), to lactating rumen-fistulated dairy cows. Buffer soluble N ranged from 36 to 802 g/kg N. There were also large differences in BSN composition. Buffer soluble protein N ranged from 0 to 874 g/kg BSN with peas and lupine seeds containing the highest amounts. There were differences (P<0.05) in the buffer soluble protein in vitro degradation rates among the feeds. Degradation rate was highest for casein (1.0 h−1) and lowest for linseed cake (0.18 h−1). Buffer soluble protein from soybean meal, peas and lupine were degraded at intermediate rates, 0.46, 0.39, and 0.34  h−1, respectively. In the in vivo experiment, soluble proteins were rapidly degraded and ruminal protein concentrations had reached baseline values within 1 h post dosing. Results were, however, compromised by very slow mixing in the rumen. The increase in ammonia N concentration when cows received soluble protein from pea, in contrast to rapeseed cake, supports the in vitro results, where peas were degraded at a higher rate (0.39 h−1 versus 0.19 h−1). Soluble proteins from different feeds were degraded at substantially different rates. Feeds high in soluble proteins, like peas and lupines, should be studied in vitro and not in sacco, since the latter method cannot measure the degradation rates of soluble proteins.